1. Field of the Invention
The present invention is broadly concerned with an improved method and apparatus for the testing of the toughness of products, particularly edible products such as bread, cheese and the like. More particularly, it is concerned with such a method and apparatus which gives accurate toughness measurements unaffected by factors such as staling, moisture content or outer crumb characteristics in the case of baked products.
2. Description of the Prior Art
It is well known that stale bread can be refreshened by heating. However, when the heating is done in a microwave oven the bread becomes tough (i.e. rubbery and hard to masticate). Several procedures to measure microwave-induced toughening of bread have been suggested. Rogers et al (1990) measured microwave-induced toughening of bread with the Instron Universal Testing Machine and the Kramer shear-compression cell attachment. They concluded that the peak force was related to organoleptic firmness and the shoulder force was related to organoleptic toughness. Marshall (1991) showed that the force deformation curves obtained from Kramer shear-compression cell analysis did not measure microwave-induced toughening of bread. The shoulder force was shown to be a combination of the force required to push the bread out of the bottom of the cell and the friction of the bread residue remaining in the cell and on the blades. The peak force was also shown to be indirectly correlated to the bread moisture content.
Dahle and Montgomery (1978) devised a method to measure bread staling which subjected a slice of bread to combined compression, shear, and tensile forces. In this method, a bread slice was centered on a platform over a cup. A disk of smaller diameter was attached to the crossarm of an Instron Universal Testing Machine. As the disk descended into the cup, the bread was deformed and ruptured. The point of bread rupture corresponded to the peak resistance and was termed "crumb strength". The distance from onset of deformation to point of peak resistance (displacement of the peak) was termed "crumb extensibility". Dahle and Sambucci (1987) applied the method to frozen bread which was thawed in a microwave oven. They noted an increase in crumb extensibility after microwave thawing. Karlsson (1991) used the method to analyze microwave reheated bread. The extensibility of the bread increased as microwave heating time increased and was selected as a measure of microwave-induced toughness.
However, it has been found that all of these prior techniques are deficient in that they fail to give a fully differentiated measure of microwave-induced toughness in bread products. These prior methods can be significantly influenced by staling, moisture content, outer crumb characteristics, or other factors not related to toughness of microwave reheated breads.
In addition, other types of products (e.g., cheese) exhibit toughness characteristics, and an accurate, differentiated method for measuring toughness in these contexts is also needed. There is accordingly a need in the art for an improved method and apparatus for the accurate determination of toughness of products, and particularly microwave reheated baked products.